Pour several samples of liquid acids and bases into disposable cups.  Using a spoon or dropper, add about a teaspoon of the pH indicator to each sample.  The acids will turn colors that are in the purple to pink range, and the bases will turn colors that are in the brown to yellow range.

SAFETY NOTE:  When experimenting with or disposing of household chemicals, be careful not to combine them -- certain chemicals can combine to produce unsafe gases, and it is always better to be safe than sorry.

Science:  Most of us know about acids and bases in the context of making pretend volcanoes.  If you combine vinegar (an acid) with baking soda (a base) you will get a great bubbling, fizzing chemical reaction.  (CAUTION:  sometimes household chemicals, when combined, make poisonous gases, so when combining your acids and bases, stick to vinegar and baking soda.)  The reaction happens as the two substances give or receive ions and neutralize each other. 

Acids typically taste sour (think of lemon juice -- though not all acids are edible!) and bases typically feel slippery (think of soapy water).  Kids probably won't understand the specifics of the molecular structure that makes up an acid or a base -- it's enough to know that sometimes things are acids and sometimes things are bases, just like sometimes things are insects and sometimes they are birds, and you can tell them apart with a pH indicator like poinsettia juice.  It is also worth noting that not all things are an acid or a base.  Many things are neither. 

Poinsettia leaves work as an indicator because they contain a particular pigment that changes color when it is mixed with an acid or a base. Purple cabbage has similar properties and can also be made into a pH indicator.  Read how